Cardiac pacemakers are medical devices, usually implantable, that can be connected to or that are permanently connected to electrode leads for delivery of electrical stimulations pulses to the tissue (myocardium) of a human heart. Dual chamber pacemakers are capable of generating stimulation pulses for the atrium and the ventricle of a human heart. Biventricular pacemakers usually are capable to stimulate at least three chambers of a human heart that is the right atrium, the right ventricle and the left ventricle.
In a dual chamber pacemaker, this is realized by placing electrodes in both the right atrium and right ventricle of the heart.
Separate stimulation pulse generators are usually provided for each heart chamber (atrium or ventricle) to be stimulated.
A control unit is triggering the generation of a respective atrial or ventricular stimulation pulse according to a pre-programmed, variable timing regime in order to provide for adequate timing of the stimulation pulses. The stimulation pulses triggered by the control unit are triggered with a controlled stimulation rate controlled by the control unit. The controlled stimulation rate may depend on the physical or mental load of a patient. Thus, the hemodynamic demand of the patient is met. Sensor means for determining the hemodynamic demand of a patient allow for rate adaptive pacing wherein the controlled stimulation rate is determined based on the hemodynamic demand of the patient as determined by the sensor means and the controlled stimulation rate is adapted accordingly. A strong hemodynamic demand leads to a high controlled stimulation rate.
Further, the controlled stimulation rate of ventricular stimulation pulses may depend on a rate of sensed intrinsic atrial contraction when atrial synchronous pacing is performed where no natural (intrinsic) ventricular contraction can be sensed within a predetermined atrioventricular interval (AVD) after a sensed atrial contraction. In such case, a high atrial rate leads to a high controlled ventricular stimulation rate.
If the heart stimulator is designed as an implantable cardioverter/defibrillator, a high controlled stimulation rate may be triggered in order to treat or prevent tachycardia or fibrillation of a heart chamber.
In order to monitor the heart chamber and thus to determine whether or not a contraction of a heart chamber has occurred a pacemaker has a sensing stage for sensing a heart parameter indicating a natural (intrinsic) or stimulated contraction of a heart chamber.
The sensing stage can be connected to an electrode placed in a respective heart chamber. A contraction of a heart chamber can be detected by evaluating electrical potentials sensed by such sensing electrode.
Alternatively, the sensing stage can be designed to response to the mechanical action of the heart. One way of detecting mechanical action of the heart is to evaluate an time course of intracardiac impedance.
Providing a sensing stage for a heart chamber to be stimulated allows for inhibition of delivery of a stimulation pulse to that chamber in case an intrinsic contraction of said chamber occurs within a respective atrial or ventricular escape interval. Such mode of pacing or stimulating a heart is called demand mode because a heart chamber only is stimulated if there is a demand whereas a stimulation pulse is skipped (inhibited) if there is no demand because the heart chamber has contracted on it's own.
Many heart stimulators are capable to perform a number of different stimulation that are characterized by a commonly known three letter code wherein the first letter designates the chamber or chamber to be stimulated like V for a ventricle to be stimulated, A for an atrium to be stimulated and D (dual) for both, ventricle and atrium to be stimulated. Similarly, the second letter characterizes the chamber or chambers the heart stimulator can pick up sensed signals from (V: ventricle, A: atrium, D: ventricle and atrium). The third letter characterizes the mode of delivery of stimulation pulses: T=triggered, I=inhibited and D=dual (T+I). A fourth letter “R” may characterize a rate adaptive heart stimulator.
Regarding application of a high controlled stimulation rate to a heart chamber, it is to be noticed that a fault in a cardiac pacemaker causing a sustained high pacing rate could lead to driving a patient's heart to a state where an ischemia develops, leading to harmful tachycardia.
In order to avoid such consequences of a pacemaker's fault cardiac pacemakers usually comprise a separate circuit with an independent oscillator for monitoring the stimulation rate and to inhibit stimulation pulses that could exceed a give rate threshold. This feature often times is called runaway protection. Pacemakers provide for features that include legitimate conditions in which temporarily violating such rate limit said by the runaway protection is necessary. For example, the delivery of a backup stimulation pulse after a non-effective stimulation pulse should occur with a rate that is higher than a certain rate limit. Another example of a feature where a higher stimulation rate is required is the delivery of a burst of stimulation pulses intended to trigger or to stop arrhythmias. In order to allow for such features, prior art pacemakers comprise means to temporarily disable or override the runaway protection. In order to reduce the probability of inadvertently activating the means for overriding the runaway protection, these overriding means are usually designed with a certain degree of intentional complexity.
It is an object of the present invention to provide a heart stimulator with improved means for runaway protection.